1. Field of the Invention
The present invention relates to monitoring the physical state and condition of a surface and more particularly to the use of second-order nonlinear optics to determine the physical aspects of a surface to a high degree of specificity.
2. Description of the Related Art
The spectroscopy of the microwave regime (electromagnetic phenomena with wavelengths from 100 centimeters to 1 centimeter) is a relatively unused means to characterize physical phenomena. Use of that spectral region is typically limited to communication applications. Material diagnostics typically focus on responses in the optical regime, with concentration in the visible (0.33 to 0.77 micron) and infrared (0.77 to 20 micron) portions of the spectrum. The exception to this is in nuclear magnetic resonance techniques, which examine the long wavelength spectral responses associated with nuclear spin flips. In fact, there are a great deal of physical excitations in that spectral region, primarily associated with molecular rotations, that could be used to analyze and diagnose the properties of a physical system. In particular, the microwave properties of a surface may prove to be very important.
In nonlinear optics or socalled wave mixing processes, outputs are produced at sum, difference or harmonic frequencies of the input(s). Using second order nonlinear optical surface spectroscopy to examine the physical properties and behavior of a surface or interface was originally proposed in the 1960""s, in xe2x80x9cLight Waves at the Boundary of Nonlinear Mediaxe2x80x9d by Bloembergen and P.S. Pershan, The Physical Review, 128, Page 193 (1962). Experimental work involving second harmonic generation was also performed. However, because lasers at the time were comparatively feeble, impractical, slow, etc., there was little subsequent work done on the development of second harmonic generation or, more generally, second order nonlinear optical (NLO) processes at surfaces until considerably later, when lasers had reached a certain level of technical maturity.
Recently, researchers have reviewed NLO processing and concluded that lasers had developed enough that they could be used for studying the physical and chemical properties of surfaces and interfaces. For example, a theoretical study of the physics of the interface, and not its engineering aspects, has been performed. See Journal of Vacuum Science and Technology B. Volume 3, Number 5, September October 1985, Pages 1464xe2x88x9d1466, Y. R. Shen, xe2x80x9cSurface Studies by Optical Second Harmonic Generation: an Overview.xe2x80x9d
However, the work conducted thus far with surface nonlinear optics has been limited to optical wavelengths. If one extends the theory to longer wavelengths ( greater than 100 microns), it turns out that the analogous processes still apply. It would be difficult to perform surface measurements using microwave sources as inputs, as high peak power coherent sources in this regime are not readily implemented and mixing with optical sources would be problematic.
In U.S. Pat. No. 5,294,289, T. F. Heinz et al. discuss the use of second harmonic generation as a means to monitor the epitaxial growth of silicon semiconductor structures in a high vacuum chamber. Specifically, they examined the spectroscopic response at the interface between the electronically active silicon and the insulative layer of calcium fluoride. By monitoring the magnitude of the resonance, they could ascertain whether the insulator was present on the surface and whether it had electronically binded to the underlying semiconductor. The system that is used examines the total intensity only of the second harmonic light that is generated and there is no discussion of the use of optical difference frequency generation between two input sources. There is also no discussion of microwave measurements.
In U.S. Pat. No. 5,623,341, J. H. Hunt discusses the use of sum-frequency generation for the detection of contamination and corrosion on engine parts. In this incarnation, one of the inputs is a tunable IR beam that is tuned to a resonance of the contamination on the surface. The efficiency of the sum-frequency process is increased (so-called resonant enhancement) when the IR beam is resonant with a contaminant. If the contaminant is not present, there is no resonant enhancement. By comparing on and off resonant signals, the presence and level of contaminant can be deduced. However, there is no discussion of applying difference frequency generation to observe a material resonance. There is no discussion of microwave measurements.
In U.S. Pat. No. 5,875,029, P. C. Jann et al. describe a versatile optical inspection instrument and method to inspect magnetic disk surfaces for surface defects. The device provides surface position information of the defects. However, the technique involves only linear optical processes. That is, the input and output light wavelengths are the same.
In U.S. Pat. No. 5,883,714, Jann et al. describe a versatile optical inspection instrument and method to inspect magnetic disk surfaces for surface defects. The device is based on interferometric measurement and detects contaminants by measuring the Doppler shift in the light that results from scanning the light onto a contaminant or defect. By scanning, the device provides surface position information of the defects. However, the technique involves only linear optical processes and senses only phase changes. That is, the input and output light wavelengths are the same.
In U.S. Pat. No. 5,898,499, J. L. Pressesky discusses a system for detecting local surface discontinuities in magnetic storage discs. The device is an interferometric detector which scans the disc in a spiral motion. Local defects cause local changes in phase, which are measured by interferometric techniques. This is a linear optical technique.
In U.S. Pat. No. 5,932,423, T. Sawatari et al. discuss a scatterometer for detecting surface defects in semiconductor wafers. This device is a linear interferometric device.
In U.S. Pat. No. 5,973,778, J. H. Hunt discusses the use of second harmonic generation for investigating molecular alignment within a thin polyimide film. The technique uses changes in the second harmonic polarization to determine surface molecular alignment. There is no discussion of difference frequency generation from the surface to be interrogated. Furthermore, there is no discussion of microwave regime responses.
In U.S. Pat. No. 6,317,514 B1, S. Reinhom et al. discuss a method and apparatus for inspecting a wafer surface to detect the presence of conductive material on the wafer. The device uses UV initiated electron emission to determine the location of conductive areas. Those areas which are metal will emit electrons. If the area, which is supposed to be conductive, is not, there will be no electron emission.
In U.S. Pat. No. 6,359,451 B1, G. N. Walimark discusses a system for testing for opens and shorts between conductor traces on a circuit board. The technique uses electron scattering to perform its diagnostics and has no optics associated with it.
The present invention is a nonlinear optical system for performing surface-sensitive microwave spectroscopic characterizations on a surface to be interrogated. In a broad aspect, the present invention includes a first optical source for providing a fixed visible input directable to a location on a surface to be interrogated. A second optical source provides a tunable visible input that is directable to the surface to be interrogated. The fixed visible input and the tunable visible inputs are alignable so that their surface locations of optical illumination overlap on the interrogated location. An output wavelength discriminator receives the reflected difference-frequency generated on the interrogated location. The output wavelength discriminator is substantially non-transmissive at frequencies higher than the difference-frequency, but substantially transmissive at the difference-frequency of the fixed visible input and the tunable visible input. The output of the output wavelength discriminator is a microwave output. Signal collection optics receive the microwave output of the output wavelength discriminator and direct the propagation of the microwave output of the output wavelength discriminator so that a collected microwave light signal is formed after propagation through the signal collection optics. A microwave detector system converts the collected microwave light signal to an electronic signal. Thus, the intensity of the difference-frequency wavelength is monitored for providing surface-sensitive spectroscopic characterizations on the surface to be interrogated.
Ordinarily, the characterization of the microwave spectroscopic properties of a material requires a source at that range of wavelengths. Microwave spectroscopy requires a microwave source which generates light in the microwave electromagnetic regime. However, when studying the nonlinear optical response of a material, resonant behavior can be achieved even if only the difference between two input sources corresponds to a material resonance. In many cases, it is difficult to generate microwave light at arbitrary wavelengths, but it is always possible to generate tunable visible light, whose wavelength can be tuned near the wavelength of a fixed visible source. In the present case, where nonlinear optical processes are being used to monitor a surface, it is important that the input sources have high peak power. It is difficult to produce high peak power optical pulses at arbitrary microwave wavelengths. Instead, difference-frequency allows the use of visible high peak power pulses, which are easier to produce, to make the same measurements.
There is additional complexity in that the detection of low levels of microwave radiation is non-trivial. Two means to accommodate detection in this regime is to either (A) combine the weak microwave signal with a strong optical beam via a nonlinear optical interaction and detect that combined light or (B) detect the microwave source directly with a specialized detector, such as a metal-oxide-metal, so called MOM, detector.
Other objects, advantages, and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.